1. Field of the Invention
This invention relates to rotational coupling devices such as brakes and clutches and, in particular, to a rotational coupling device having structure to reduce flux leakage from the magnetic circuit used to engage the device.
2. Discussion of Related Art
Rotational coupling devices such as clutches and brakes are used to control transfer of torque between rotational bodies. One type of conventional device is illustrated in commonly assigned U.S. Pat. Nos. 5,119,918, 5,285,882, 5,971,121, 7,493,996, 7,527,134, and 7,732,959 and U.S. Published Patent Application Nos. 2006-0278491, 2009-0229941 and 2010-0116616, the entire disclosures of which are incorporated herein by reference. This device includes a rotor that is coupled to a shaft for rotation with the shaft about a rotational axis. A field shell is also disposed about the axis on one side of the rotor and is fixed against rotation. The field shell defines radially spaced, axially extending inner and outer poles between which an electrical conductor is disposed, facing the rotor. A brake plate is coupled to the field shell and axially spaced from the field shell. The brake plate is disposed on a side of the rotor opposite the conductor. An armature coupled to a torque transmission member such as a pulley is disposed on the same side of the rotor as the brake plate and is disposed axially between the rotor and the brake plate. The armature is coupled to the torque transmission member by a plurality of leaf springs. Energizing the conductor produces a magnetic circuit in the field shell, rotor and armature that draws the armature into engagement with the rotor and couples the shaft and torque transmission member together for rotation. Upon deenergization of the conductor, the leaf springs draw the armature out of engagement with the rotor and into engagement with the brake plate to brake the armature and torque transmission member. Permanent magnets may also be coupled to the brake plate and used to create another magnetic circuit between the brake plate, the field shell and the armature to assist the leaf springs in braking the armature and torque transmission member.
The magnetic circuit between the field shell, rotor and armature used to engage the coupling device may be weakened by leakage of magnetic flux along various paths. In particular, magnetic flux may leak from the armature and/or brake plate to the adjacent torque transmission member. The flux then travels radially inwardly through all available paths which could include a supporting bearing and possibly the shaft on which the coupling device is mounted before returning to the rotor. This presence of this flux pathway (or leakage circuit) drains magnetic flux from the rotor/armature interface thereby reducing the flux density at the interface and the attractive force between the rotor and armature. As a result, the coupling torque and the wear life of the coupling device is reduced.
The inventors herein have recognized a need for a rotational coupling device that will minimize and/or eliminate one or more of the above-identified deficiencies.